FIG. 19 shows the side view of an optical pick-up device comprising a conventional optical diffraction grating element 22. A laser beam emitted from a semiconductor laser 21 is focused on a recording medium 25 through an optical diffraction grating element 22, a collimating lens 23 and an objective lens 24. The reflected light from the recording medium 25 passes through the objective lens 24 and the collimating lens 23 and then is diffracted by the optical diffraction grating element 22 so as to be focused on two points on a light receiving element 26.
The optical diffraction grating element 22 is comprised of, as shown in FIG. 21(a), two regions 22a and 22b having different grating pitches. The values of grating pitches d.sub.21 and d.sub.22 in the regions 22a and 22b are determined such that lights, which are incident on the regions 22a and 22b after being reflected at the recording medium 25, are converged to two points on the boundaries of the light receiving element 26 as described above.
With reference to the perspective view of FIG. 23, the forming process of the focused light spots will be explained in more detail on assumption that the grating pitch d.sub.21 &lt; the grating pitch d.sub.22. The 0th-order diffracted light of a laser beam (indicated by the solid line in FIG. 23) incident on the region 22a after being emitted from the semiconductor laser 21 is converted to a parallel beam at the collimating lens 23. Thereafter, the light beam passes through the objective lens 24 so as to be focused on a point on an optical axis R of the recording medium 25 and reflected thereat. The reflected light is projected back to the region 22b through the objective lens 24 and the collimating lens 23. One of the first-order diffracted lights produced at the region 22b is focused to form a focused light spot S.sub.21 on the light receiving element 26. On the other hand, the 0th-order diffracted light of a laser beam (indicated by the chain double-dashed line in FIG. 23) incident on the region 22b having a relatively large diffraction pitch d.sub.22, after being emitted from the semiconductor laser 21 is reflected back to the region 22a whose diffraction pitch d.sub.21 is relatively small, through the objective lens 24 and the collimating lens 23. One of the first-order diffracted lights produced at the region 22 a, which has a relatively large diffraction angle, is focused on the light receiving element 26 to form a focused light spot S.sub.22.
The focused light spots S.sub.21 and S.sub.22 and the emission center of the semiconductor laser 21 are aligned on the same plane and the direction of the array (indicated by the arrow Q in FIG. 23) is parallel with the track direction of the recording medium 25.
The light receiving element 26 is equally divided into four light receiving sections 26a, 26b, 26c and 26d as shown in FIG. 20. On the basis of output signals Sa to Sd released from the light receiving sections 26a to 26d by Push-Pull Method, a radial error signal RES is obtained by the following equation: RES=(Sa+Sb)-(Sc+Sd). The objective lens 24 is actuated such that the value of the radial error signal RES becomes zero. As a result, the laser beam emitted from the semiconductor laser 21 is accurately focused on the data recording track of the recording medium 25. When the radial error signal RES is zero, the center of the focused light spot S.sub.21 is positioned on the boundary of the light receiving sections 26a and 26b, and the center of the focused light spot S.sub.22 is positioned on the boundary of the light receiving sections 26c and 26d.
It is well known that a teeth-saw-configuration, in which longer one of two inclined surfaces composing a groove has a predetermined groove tilt angle .theta., is most suitable for the configuration of the optical diffraction grating element 22, since a diffraction grating element in this configuration possesses a particularly high diffraction efficiency. For fabricating a diffraction grating element in the teeth-saw-configuration, Ion Beam Etching Method is widely known. In this method, a material such as GaAs and PMMA, that can be etched by ion beams at faster speed than photoresist 27 to be covered thereon, is used as a substrate 28 to be processed. The photoresist 27 covered on the substrate 28 is masked with a grating that is produced by a holographic method, and processed to be in the condition shown in FIG. 22(a). Thereafter, ion beams are obliquely projected onto the substrate 28 at a fixed incident angle so that the incident direction of the ion beams is orthogonal to the direction in which the photoresist 27 extends. The above ion beams are made from e.g. Ar gas. As shown in FIG. 22(b), the photoresist 27 formed on the substrate 28 has pinning functions, and the substrate 28 is etched with the photoresist 27 serving as a vertex.
In the case the ion beams are made from Ar gas, the section of the diffraction grating finally obtained is as shown in FIG. 22(c): i.e., the grating is composed of (i) an inclined surface having a groove tilt angle .theta. which has a certain relation to the incident angle of the ion beams, and (ii) an inclined surface perpendicular to the first inclined surface.
In the process of fabricating, by Ion Beam Etching Method, the optical diffraction grating element having different-pitched regions by which incident light thereon is diffracted in different directions and focused, diffraction gratings having different pitches are manufactured at the same time in view of the simplified manufacturing process, productivity and low cost. With the conventional processing method, diffraction gratings having different pitches d.sub.21 and d.sub.22 are obtained by either of the following ways: (i) the diffraction gratings are made identical in the groove depth t but different from each other in the groove width L and the width M of the land that is a flat surface between the successive grooves (see FIGS. 21(b) and 21(c)); and (ii) the diffraction gratings are made different from each other in the groove depth t and the configurations thereof are made analogous (see FIGS. 21(d) and 21(e)). The grating pitches of the diffraction gratings are, thus, made different from each other by varying the groove configuration or groove size. This, however, causes the regions 22a and 22b of the optical diffraction grating element 22 shown in FIG. 21(a) to have different diffraction efficiencies. As a result, a difference is caused between the light amounts of the two focused light spots S.sub.21 and S.sub.22 which are formed on the light receiving element 26. This is because the focused light spot S.sub.21 for example is formed by light undergone 0th-order diffraction at one region (e.g. the region 22a) and first-order diffraction at the other region (e.g. the region 22b ). Therefore, it is understood that the light amounts of the focused light spots S.sub.21 and S.sub.22 respectively depend on the product of the 0th-order diffraction efficiency at one region of the optical diffraction grating element 22 and the first-diffraction efficiency at the other region thereof (this product is hereinafter referred to as "optical efficiency"). If the lights amounts of the focused light spots S.sub.21 and S.sub.2 differ from each other, there arises such a drawback that even when the focal point of the objective lens 24 is accurately positioned on the data recording track of the recording medium 25, the radial error signal RES does not become zero and therefore an offset is required.
There is still another disadvantage that a laser beam cannot be focused on the recording medium 25 in good condition because the laser beam is diffracted at the regions 22a and 22b with different diffraction efficiencies, in the forwarding path.
Further, unless the optical diffraction grating element 22 is designed such that the optimum 0th-order diffraction efficiency of one region and the first-order diffraction efficiency of the other region are optimum, the optical efficiency (the product of them), namely, the light amounts of the focused light spots S.sub.21 and S.sub.22 formed on the light receiving element 26 will be low. If such an optical diffraction grating element 22 is used for e.g. an optical pick-up adapted to a video disk player wherein analog signals are recorded and reproduced by FM modulation, a S/N ratio necessary for the processing of video signals cannot be obtained.
If the optical diffraction grating element 22 is used for an optical pick-up adapted to a compact-disk player, the signal strength depending on the light amounts of the focused light spot S.sub.21 and S.sub.22 will be decreased. In order to solve this problem, i.e., a decrease in the signal strength, the following measures have been conventionally taken: the sensitivity of the light receiving element 26, the amplification factor of electric signals, and/or the power of the semiconductor laser 21 are increased. Consequently the members of the optical pick-up other than the optical diffraction grating element 22 are overloaded in order to obtain high quality read-out signals. This causes an increase in the production cost.
Furthermore, in the case the optical diffraction grating element 22 is adapted to an optical scan device in which a light source for emitting a laser beam and optical diffraction grating element are provided so as to be relatively movable to each other, and in which linear scanning operation is performed by diffracting a laser beam from the light source by the optical diffraction grating element, the diffraction efficiency is also varied depending on the regions of the optical diffraction grating element 22. This makes it impossible to perform scanning operation with a constant light amount.